Single-stage flowing liquid film impactor for continuous on-line particle analysis

Pergamon

PII: S0021-8502 (97) O0013-x

J. Aerosol Sci. Vol. 28, No. 8, pp. 1539 155L 1997 ,~, 1997 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0021-8502/97 $17.00 + 0.00

S I N G L E - S T A G E F L O W I N G L I Q U I D F I L M I M P A C T O R F O R C O N T I N U O U S O N - L I N E P A R T I C L E A N A L Y S I S

Anders Karlsson,* Knut Irgum*t and Hans-Christen Hansson *

* Depar tment of Analytical Chemistry, University of Ume~, S-901 87 Ume~., Sweden Institute of Applied Environmental Research, Air Pollution Laboratory, Stockholm University,

S-106 91 Stockholm, Sweden

(First received 8 May 1996; and in final form 21 January 1997)

Abst rac t - -A single-stage "wet impactor" is presented, where the impaction occurs on a regenerated water surface. The developed impactor is equipped with an impaction liquid support plate of etched glass and a drain spout providing a continuous liquid flow covering the impaction area. Subsequent t ransport of the impaction liquid makes an on-line determination possible. With multiple nozzles (74 holes, 0.3 m m i.d.) and an air flow of 10 l/rain the cut-off was determined to 0.41 + 0.02 pm. The impactor was also investigated for its particle loss. The cut-off function, regarding the consequences of letting impaction occur in a liquid film is discussed and compared to conventional impactors. The analysis technique was tested in an ambient air measurement study with an ion chromatograph attached to the sampling system. © 1997 Elsevier Science Ltd. All rights reserved

I N T R O D U C T I O N

Collection of particles for subsequent chemical determination is often performed with inertial impactors, impingers, or filters. In contrast to filters and impingers which capture the total mass of particles of all sizes, inertial impactors are widely used in situations where specific particle size information is desired. In combination with various analysis techniques such as, ion chromatography, atomic absorption spectrometry, and PIXE, impactors have been utilised for chemical analysis of the atmospheric particle phase (Appel, 1993). Impac- tion is furthermore an especially suitable technique for sampling of bioaerosols, e.g. viruses, bacteria and fungal spores (Nevalainen et al., 1992). The theoretical background, which has been investigated by Marple (1970) and Radar and Marple (1985), is well understood, which facilitates impactor construction for special applications.

Impactors in their present design are inherently discontinuous sampling devices, although a few devices have been developed for achieving temporal-resolved information of aerosols. An electronic cascade impactor was introduced by Tropp et al. (1980) for measuring the particle size distribution. The sampled particles were uni-polar charged previous the cascade impactor and the associated current to the number of particles collected on each stage was measured. Keskinen et al. (1992) later improved this idea by extending the measuring range to smaller particle sizes by a low pressure impactor. In an another attempt to solve the problem of time-resolved sampling, Hansson and Nyman (1985) constructed an microcomputer-controlled automated aerosol sampler. A sequential sampling was achieved by letting the sample aerosol be collected on an impaction plate and a filter mounted on an movable concentric cylinder. The only work found in the literature where impaction has been coupled to on-line instrumentation for chemical analysis is the work by Sneddon (1984), who presented a system for collection and determination of particle-bound lead. The particles were sampled by impaction directly onto the surface of a graphite furnace, from where they were subsequently atomised and determined by atomic absorption spectrometry in a semi-continuous fashion.

t Author to whom correspondence should be addressed.

1539

1540 A. Karlsson et al.

Automatic size segregated sampling techniques for characterisation of the ionic composi- tion of particles has so far never been shown in the literature. Instead, discontinuous techniques, involving manual washing of impaction surfaces with subsequent discrete analysis, are used for characterisation of impactor samples. These steps are frequently associated with troublesome and time-consuming sample handling, likely to cause biased results due to contamination risks or alteration of the sample constitution by, e.g. oxidation or biological activity. Recently, Simon and Dasgupta (1995) described an aerosol sampling system based on the wetted wall parallel plate diffusion denuder (PPDD) for gas sampling with a particle collection system (PCS) to collect the particle phase. The particle fraction was allowed to grow in a super-saturated water vapour after the removal of water-soluble gases, and was accumulated in a gas/liquid separator. The liquid could be collected from the bottom tip of the separator and automatically pumped to ionic determination and the collection efficiency was found to be 98 100% for experimental aerosols. Buhr et al. (1995) later described, in many respects, similar technique where the particles were collected on a continuously wetted flit, which in the same way as the PCS was connected downstream a wetted wall denuder. Both techniques offer low detection limits, and a high temporal resolution, but are not able to size segregate the particle fraction.

In this work we present a modified single-stage impactor, which we term wet impactor (WI), where the impaction surface consists of a continuously regenerated water film. The impaction occurs in the film, where the water-soluble fraction of the particles will be extracted while the sample is being transported to the analysis instrument for further sample preparation and determination. The performance and the cut-off profile of the WI has been compared to conventional impactors and parameters related to the formation and susten- ance of the impaction liquid film were investigated and are being discussed. Its utility for ambient air measurement has been demonstrated by an on-line determination of particle- bound inorganic ions, using an enrichment step followed by ion-chromatographic determination.

E X P E R I M E N T A L SECTION

W e t impac tor cons t r uc t i on

A schematic drawing of the WI is shown in Fig. 1, with a detail on the impaction plate assembly, consisting of the impaction liquid support plate with its drain spout, the nozzle plate, and the impactor housing. The impaction liquid support plate was cut from ordinary window glass with dimensions of 50 x 50 mm and a thickness of 3 ram. A 1 mm deep by 1 mm wide circular channel with 35 mm outer diameter was cut into the glass plate. To impart hydrophilic properties to the surface of the impaction area, etching with a saturated solution of ammonium hydrogen difluoride was carried out for 15 min at room temperature, with the area outside the circular channel masked with Parafilm M (American National Can, Greenwich, CT). The impactor plate was thereafter thoroughly rinsed with water and the Parafilm peeled off. Two 1.6 mm diameter holes were then drilled with diamond bits; one through the centre of the etched impaction area, to fit the liquid drain spout described below, and the other from the opposite side of the circular channel, so that a 0.8 mm orifice opened in the bottom of the cut. The sketch in Fig. 2a shows the impaction liquid support plate from above.

Several liquid drain spouts were made from poly(propene), poly(ether ether ketone) (PEEK) and poly(vinyl chloride) (PVC) by machining the tips of short rods of respective material to 1.6 mm outer diameter and drilling a 1 mm diameter dead end internal channel from the opposite end. This was followed by drilling (a) perpendicular 0.4 mm diameter hole(s) 0.5 mm from the end of the narrow tip. During the initial tests two or four holes were used, but the final version was made from PVC and had only one hole. The liquid drain spout was placed in the centre hole of the impaction liquid support plate with its butt end touching the nozzle plate, thereby providing a defined inter-plate distance. The protrusion of the liquid drain spout above the impaction liquid support plate was 2.8 ram. The

Continuous on-line particle analysis 1541

j j z

r

,÷ " B

G

I

I

I q , ¢

Fig. 1. Assembly drawing of the wet impactor. The supporting base (A), equipped with three screws (B) for vertically and horizontally adjustment of the impaction liquid support plate (C). Impaction liquid drain spout (D), fitting for the supply of impaction liquid (E), the nozzle plate (F) and the impaction liquid film (G). The silicone tubings for the impaction liquid flows (H) were connected

through air-tight bulkhead fittings on the impactor body.

a) O b) Fig. 2. Sketch (a) showing a top view of the impaction liquid support plate and (b) the impaction

liquid film on the impaction liquid support plate with the drain spout.

impaction liquid, which was deionised water, was drained from the liquid drain spout and supplied from the hole in the circular channel by a peristaltic pump (Gilson Minipuls 3, Villiers-Le-Bel, France) at a flow rate of 0.6 ml min- 1. A pump tubing with twice the inner diameter of the supply tube was chosen for drain suction so that the total flow rate of the drain became higher than the liquid flow, and an air/liquid segmented flow was consequently established in the impactor liquid drain. This was found to be necessary for obtaining a stable liquid flow on the impaction area. The impaction liquid film thickness was roughly estimated by a slide-calliper to approximately 1 mm. Figure 2b is representa- tive of the impaction liquid film applied on the impaction liquid support plate during operation. The cut-off characteristic for single round jet impactors is described by Stokes number (St); coherely St = 0.48 corresponds to the dso value according to Marple (1970). From the equation

St = ppd2 UCc 9qOh ' (1)

where the parameters are particle density (pp) and viscosity of air (r/), particle diameter (dp), jet velocity (U), nozzle diameter (Dh) and the slip correction factor (Co), the cut-offdiameter could be calculated. The accuracy of the equation is dependent of the Reynolds number (Re)


of the nozzle and the S / W ratio. Re is calculated from

Re - pg UDh (2)

where pg is the air density. S and W correspond to the distance between the nozzles and the impaction surface (S) and the diameter of the jet (W), respectively. S in the WI is defined as the distance between the impaction liquid surface and the nozzle plate.

The impactor was calculated to give a cutoffpoint at 0.5/~m with an air flow of 10 1 min- 1 using a nozzle plate comprising 74 holes, each 0.3 mm in diameter. Poly(methacrylate) was chosen as material for the nozzle plate, as well as for the entire impactor body in order to be able to observe the behaviour of the liquid film during operation. The nozzle plate consisted of a circular plate where the central circular piece, 50 mm diameter, had been cut out and recessed 12 mm. A circular area with 35 mm diameter was milled into this recessed section, so that the material thickness was 1.0 mm. Within this recessed area the nozzles were drilled with evenly spaced holes in four concentric circles of diameters 9 mm (10 holes), 15 mm (16 holes), 21 mm (24 holes) and 27 mm (24 holes). A detail of the nozzle arrangement over the plate is shown in Fig. 3. The nozzle plate porosity (Po) was calculated to 0.5% and is defined as

N W 2 Po - 2 , (3)

Dr

where N is the number of nozzles, W the nozzle diameter and Dc the diameter of the area within which the nozzles are placed. Re for each nozzle was calculated to be 630.

Attempts to reduce the cut-offdiameter to 0.2 ~m was performed with a nozzle plate with 43 nozzles with i.d. 100/~m. To obtain the necessary jet velocity an air flow of 1.0 lmin-1 was drawn through the impactor corresponding to a Re of 325 for each nozzle. The nozzle plate was mounted into the impactor housing in the same manner as the original nozzle plate, and placed as close as possible to the impaction liquid surface, leaving an air gap of only approximately 0.5 mm. The corresponding S / W ratio was 5. In this set-up no impaction liquid drain spout was used and therefore the incoming air was humidified to prevent the impaction liquid from evaporating.

Cut-off calibration

A TSI (St. Paul, MN) model 3076 collision atomiser spraying a 1% ammonium sulphate solution, followed by dry air addition, was used to generate a polydisperse aerosol for the

Fig. 3. The nozzle plate and the arrangement of nozzles. Porosity = 0.5%. Outer nozzle array diameter is 27 mm.


impactor calibration step. The aerosol thus generated had its count median diameter at about 50 nm with the majority of the particles below 0.2/~m. A pre-impactor with cut-off at 1/~m prevented larger particles from entering the system, and the monodisperse test aerosol was produced by classification of the polydisperse aerosol in a TSI 3070 differential mobility analyser (DMA). Two condensation particle counter (CPC, TSI 3020) were used to monitor the particle concentration up-stream and downstream the impactor, respectively. The CPCs where checked against each other and compensated for their relative deviation in measured particle concentration. The calibration was performed at 1013 hPa barometric pressure with the air flows through the impactor and CPCs adjusted by calibrated mass flow controllers. After the experimental part of the calibration, calculations were made to compensate for the entering of multiply charged particles into the calibration system that have the same electrical mobility as the desired single-charged particles in the test aerosol, which otherwise would result in a biased calibration. The experimental set-up is shown in Fig. 4.

Particle loss

To verify the extent of particle loss in the nozzles, a test aerosol of the same type used in the cut-off determination experiment was sampled for 2 h at the working flow rate of 10 lmin-1. The impactor was coupled to the ion chromatograph, which continuously measured the sampled fraction of the aerosol, while a filter pack was placed downstream the impactor to collect the non-impacted particle fraction. After the sampling period the filter was analysed for its sulphate content, and the total amount of impacted sulphate was calculated from the on-line ion chromatographic measurements. The nozzle plate was thereafter rinsed with water and the sulphate content in the rinsing solution was determined. The amount of sulphate found on the nozzle plate was thereafter compared to the total amount of sulphate found on the back-up filter and in the impaction liquid.

Comp. Air

C

Air Comp. Air

Waste

To pump

~ ) Comp. Air

To pump

To pump

Fig. 4. Experimental set-up for the wet impactor system (marked by numbers) and devices for cut-off calibration (marked by letters): pump (1), mass flow controller (2), filter (3), wet impactor (4), peristaltic pump (5), liquid bottle (6), condensation particle counter (A), electronic pulse counter (B), neutraliser (C), differential mobility analyser (D), pre-impactor (E), atomiser (F), pressure meter (P).


Gas absorption

To check for gas absorption in the impaction liquid, a SO2 standard gas generated from a permeation tube was used. The permeation rate was gravimetrically estimated to 6.40+0.23ngmin -1 at a thermostatted temperature of 21.1°C. A concentration of 0.25 ppbv was drawn through the impactor at the design flow rate of 101min 1 The impaction liquid was collected and 0.3 ml of 0.1 mM aqueous hydrogen peroxide was added, to oxidize sulphite to sulphate. Determination of sulphate was thereafter carried out by ion chromatography. To calculate the absorption ratio, the amount of sulphate in the impaction liquid was compared to the total amount of sulphur dioxide that passed through the impactor. The same procedure was also performed with zero gas instead of SO2 standard gas for background check.

Zero air supply

Compressed air delivered from the house supply was purified by passage through two entrained columns (800 mm by 35 mm i.d.) containing soda lime (BDH Chemicals, Poole, England) for removal of acid gases and activated charcoal (Kebo Lab, Stockholm, Sweden) to eliminate organic material. It was thereafter filtered through a 1.6 pm glass fibre filter (Whatman, Maidstone, England) to prevent particles from entering the system.

Reagents and solutions

All water, including the feed to the liquid impactor, was purified by Super-Q equipment (Millipore, Bedford, MA) and had a conductivity less than 60 nS cm-1. The ammonium sulphate (Merck, Darmstadt, Germany) used in the aerosol generation and the sodium sulphate (Merck) and sodium nitrate (Merck) for the ion chromatography standards were of p.a. grade. Stock solutions were stored in a refrigerator, and trace standards were prepared daily.

Ion chromatography (IC) and enrichment

In experiments where ion chromatography was used for evaluation, a QIC Ionchrom Analyzer (Dionex, Sunnyvale, CA) retrofitted with a Valco pneumatically operated six-port injection valve (VICI, Schenkon, Switzerland) was used. Samples of 100/21 volume were injected on a Dionex HPIC AS5A separation column, using 25 mM NaOH, prepared from carbonate-free NaOH stock (Titrisol 0.1 N, Merck), as eluent. When the ion chromatograph was implemented in the on-line WI system, the NaOH eluent was diluted to 17 mM and the sample loop replaced by a 7.7 mm by 2.9 mm i.d. PEEK enrichment column (Jour Research, Onsala, Sweden) packed with a porous methacrylate quaternary ammonium anion exchange resin (HEMA-1000 QL, 60 ~tm, Tessek Ltd, Prague, Czech Republic) for enrichment of the anions dissolved in the impaction liquid. The Dionex micro-membrane suppressor was replaced by a strong cation exchange membrane reactor (SciTech, Ume~, Sweden; Model 1120-100) and used in conjunction with an auto-regenerating system (SciTech) in order to achieve continuous operation at low background conductivity. Baseline drift due to atmospheric carbon dioxide contamination of the eluent was prevented by connecting a soda lime trap to the eluent bottle.

Sample handling

The impaction liquid was pumped to a 10ml sample holding vessel made from a poly(propene) syringe. This vessel was fitted with a machined PVC cap that had a small diameter hole for ventilation. A pneumatically operated six-port injection valve with Tefzel rotor seal (Rheodyne model 9010, Cotati, CA), controlled by a Chron-Trol Timer (Lind- burgh Enterprises, San Diego, CA), was used to direct the liquid flow. After a specified time


the valve was switched, whereby the container was drained and the liquid pumped to the enrichment column. Thirty seconds before sample injection the sample handling valve returned and the enrichment column became filled with Super-Q water to prevent a sudden pressure drop in the chromatography system during the injection. The pneumatic valve returned thereafter and the holding vessel was filled with a new sample.

Demonstration of the system

The set up shown in Fig. 5 was used to determine the performance of the WI in an ambient air measurement. A laboratory-buil t cyclone with a cut point of 3/~m at 10 1 min was used to pre-separate the coarse mode of the particle phase from the sampled air before it was allowed to enter the system. A train of denuders, the collection efficiency of which had previously been determined (Ganko, 1994), was used to quantitatively trap the gaseous HNO3 and SOz before the sample aerosols were entering the impactor. Two of the three denuders were prepared with sodium carbonate (soaking in 1% NazCO3 in 50% aqueous methanol, followed by drying by a nitrogen gas flow) and one with sodium chloride (prepared in the same fashion from 0.5% NaC1 in 90% aqueous methanol). A sample from the WI was injected every 16 min, using liquid collection and enrichment periods of 9 and 7 min, respectively. Calibration of the ion chromatograph was carried out before and after the measurement period by pumping a known volume of standards through the enrichment column. The mass flow controller was adjusted to the ambient air pressure to ascertain a correct volumetric impactor flow.

R ES ULTS AND D I S C U S S I O N

Arrivin9 at an impactor plate design

The most critical part in the design of the WI turned out to be the establishment of an impaction surface that could be flooded with water in a horizontal position without film break-up or uneven film thickness due to surface tension forces. During our initial experiments, we first tested a round glass disc, whose surface and edges were modified in the same manner as the hydrophilic area of a PPDD, i.e. coated with silicon particles (Simon and Dasgupta, 1993). Impact ion liquid was supplied by a peristaltic pump through a hole in the centrum of the plate, and the intention was that it should flow evenly over the impaction surface, to be collected in a peripheral channel formed by mounting the plate in a well with outlets for the WI effluent. This did not work satisfactorily, as the water film tended to build

8

Waste 6

Fig. 5. The experimental set-up for ambient air measurements: cyclone (1), denuder train (2), wet impactor (3), peristaltic pumps (4), reservoir for impaction liquid (5), pneumatic valves (6), container

for sample liquid (7), enrichment column (8).


up at the edges due to surface tension effects. This in turn resulted in a discontinuous siphoning over the edge and consequently an uneven flow and film thickness. Rounded edges of varying radii were tested in attempts to alleviate this, but without significant success.

In an attempt to "force" the impaction liquid over the edge, we then designed a liquid distributor to be placed in the centrum hole, with four, and later two nozzles, that spouted water gently over the impaction liquid support plate. This did, however, not improve the flow properties and a new impaction liquid support plate was therefore constructed, which became the final design shown in Fig. 2. The flow direction was still from the centre to the edge, but a shallow peripheral channel was used to collect the impaction liquid after passage across the collection surface. A more stable flow was accomplished, although some fluctuation in film height was still evident. This emanated from the exit hole, which emptied the collection channel in a discontinuous way. The solution to this was simply to reverse the flow direction, letting the impaction liquid flow from the peripheral channel to the central drain spout, which was then used as a level-adjusting collector, rather than for spouting. The two-hole drain spout was later abandoned for a single hole version, operated with suction at a rate higher than the feed rate of the impaction liquid. The single hole drain spout thus draws some air in addition to the supplied liquid, thereby ascertaining that the impaction liquid film thickness is maintained at a constant level.

The choice of material for the drain spout also required some consideration. The initial drain spout, designed as a spouting nozzle rather than as a drain, was made from poly(propene). When used for suction, this drain spout proved to be too hydrophobic, resulting in repulsion of the water film from its surface. We therefore tested drain spouts made from PEEK and PVC, whereof only PVC resulted in a meniscus that "climbed" up the drain spout. It was thus selected, although its chemical inertness may be unoptimal. With this final design, no visible unevenness was seen in the area of the film where impaction would take place. The thickness of the impaction liquid film was estimated to be 1 ram, which corresponds to a volume of approximately 1 ml of impaction liquid on the glass surface.

Establishing the impactor operating parameters

According to Marple (1970) and Radar and Marple (1985), the most important parameters in the impactor design procedure besides the Stokes number (St) are the S/W ratio and the Reynolds number (Re). The S/W ratio should by design be between 0.5 and 5 for Re between 500 and 3000 for a predictable impactor cut-off. Therefore, a stable level of the impaction surface is of great importance, and only a slight fluctuation can be allowed without affecting the impactor characteristic. With the drain spout made of PVC a visually stable level of the impaction liquid surface was achieved with an excellent long-term stability of the fill-and-drain function.

Due to the principle of letting the particles impact in a liquid, we were limited by the effect of the jets on the water film. Preliminary experiments showed that the use of one or only a few jets with comparatively large diameter required jet speeds so high, that the water film was torn apart. This eventually resulted in dry spots in the impaction area, which would have obviated the whole concept. If a multitude of very small diameter nozzles (50/~m) were used instead, the nozzle plate-to-film distance would become very small to fulfil the demands of an S/W ratio 0.5-5. The jets cause a ripple in the impaction liquid film due to Rayleigh instability, and when the distance became too small there was an increased risk of contact between the impaction liquid film and the nozzle plate, with disruption of the impaction liquid film as a consequence. A reasonable compromise was the final design, which involved 74 holes with 0.3 mm diameter, placed in concentric circles evenly distrib- uted over the impaction surface. To ascertain that an essentially level part of the film was used for impaction, regions near the edge and the central liquid drain spout were excluded when the hole pattern of the nozzle design was laid out.

The porosity of the multi-nozzle plate was chosen to be 0.5%. A higher porosity was deemed impractical since the cross-flow effect, which is caused by gas escaping in an


outward direction from the inner jets would deflect the outer jets, is minimised at low porosities. Gudmundsson et al. (1995) noticed that a porosity of 1.3% gave a cut-off curve similar to a single-nozzle impactor, whereas with a porosity of 5% the cross-flow effect was significant and the efficiency declined for particles above the cut point.

Impactor calibration

Results from the impactor calibration, Fig. 6, show that the above design resulted in a cut-off diameter of 0.41 _ 0.02 pm instead of the calculated 0.51/zm at 10 1 min- 1. The

calibrated cut-off diameter corresponds to a Sx/~5o = 0.41 + 0.02, which differs from the

theoretical value S~x/~5o = 0.48 (Marple, 1970). A reasonable explanation for this deviation is that each jet creates a cavity in the water film, resulting in a jet-to-surface angle of less than 90 ° at the point of impaction. Particles would then experience a greater inertial momentum, which increases the likelihood of impaction. No experiment was, however, carried out to verify this hypothesis.

The calibration diagram (Fig. 6) shows that the impaction curve never reached 100% efficiency for the largest particles. The DMA-based calibration set-up was not capable of generating particles larger than 1/~m due to the physical dimensions of the DMA, and calibration was thus not performed for particle diameters above that limit. The lack of efficiency for larger Stoke numbers and the flattened shape of the calibration curve can reasonably be a consequence of a high S/W ratio. An S/W ratio greater than 5 can affect the slope of the calibration curve negatively for multi-nozzle impactors, especially for Re higher than 400, as shown by Gudmundsson et al. (1995). By calculating the geometrical standard deviation (%) of the differential collection efficiency curve, the deviation from an ideal impactor function, where % is 1, can be determined. A less resolving impactor with a flatter efficiency curve, will increase the % -value. The geometrical standard deviation is calculated as

~g = . ~ (4) dp.o.16

assuming a log-normal distribution (Mercer, 1976), where d v 0.s4 and dp o.16 are the particle size at 84 and 16% collection efficiency, respectively. The experiments performed by Gudmundsson et al. (1995) at S/W ratios of 5.3 and 8.8 (Re = 300) resulted in only a small change in O-g values, 1.31 and 1.33, respectively, and indicates slightly less steep impaction curves compared to their experiments carried out at lower S/W ratios but at the same Re. Moreover, they also showed that an increase in Re from 300 to 500 resulted in an increase in

100

90

80

7O "~ 60

so

• = 40

-~ 30 0 L) 20

10

jd i l

,.) #

0 0.2 0.4 0.6 0.8 DJlttm

; " ~ Average ~" . .D . .#1

. . ~ . . # 2

--o.- .#3

Fig. 6. Results from the cut-off calibration. The average curve is calculated from three measurements.

1548 A. Kar l s son et al.

Crg to 1.45 and 1.56, respectively, at the previously in mentioned S/W ratios. In our design we determined ag to be 1.47 _ 0.02, and estimated the S/W ratio to 5, using 1.5 mm as the value for S. However, in the cavities that formed where the jets impact the surface, the nozzle plate-to-impaction plate distance increased, and consequently, the effective S/W ratio became greater. A Re of 630 and an S/W ratio > 5 can be an explanation for the large ~rg for the WI. The shape of the curve can be compared to the one determined by Gudmundsson et al. (1995) at an S/W ratio of 8.8 where a similarity can be noticed, that indicated an increase in the S/W as an effect of the jet influence on the impaction liquid surface (Fig. 7).

For further comparison of the function of the WI, we can compare our results with those of Rubow and Hillamo (1994) who calibrated a multiple impactor of a design similar to ours, except for a higher Re (80 nozzles, 425 ttm i.d., Re = 1340, S/W = 1.41). A manifest difference was evident, with their design showing a steeper collection curve corresponding to a better size-resolving collection (Fig. 7). Their ~rg was estimated to 1.15. This further indicates that the lack of efficiency is an effect of the wet impaction principle and a large S/W ratio, and not a consequence of the nozzle design.

Particle loss

Loss of particles due to their contact with the nozzles must be considered in all impactor designs. Cushing et al. (1979) investigated the wall losses of five impactors and found a variation between 2 and 15% for particle diameters between 1 and 3 ttm. They stated that the majority of losses occurred on the nozzles and only a small amount could be derived to losses on other parts of the impactor. In addition to inertial losses, we would in this particular design also have to consider losses due to electrostatic deposition caused to build-up of static electricity, since we had constructed the nozzle plate from dielectric poly(methyl methacrylate). However, our experiments indicate that losses in the nozzle plate were 5.0 + 1.4% (n = 7; 95% confidence interval) of the total amount of sulphate from test aerosol passing through the impactor. This nozzle loss is not widely different from those typically obtained with conventional impactors manufactured from conducting materials.

Gas absorption

Although the impaction surface area is relatively small, 49% of the sulphur dioxide added to the gas phase was absorbed in the impaction liquid film when operated at the design gas flow rate of 101min 1. When using the diffusion-based Gormley-Kennedy equation for P P D D presented by Simon and Dasgupta (1995) and considering each nozzle to represent

100

9 0

~ 8O

7o e=

• ra 6 0

so

.~ 40

30

20 l0

0 0.1

L

l I

J . , . y, I

0.3 0.5

f j o ° ~ °

. f f

/

/ - - _ - Marple /

/ - . . . . . Gudmundsson et al. (S/W = 5.3)

,, /" Rubow and Hillamo

// . ! "/ Wet Impactor

. . . . Gudmundsson et al. (S/W = 8.8)

I I 0.7 0 .9 1.1

Fig. 7. C o m p a r i s o n of efficiencies for different impactors : G u d m u n d s s o n et al. (1995), and Rubow and H i l l amo (1994), a theore t ic cut off curve ca lcu la ted by Marp l e (1970), and the current wet

impactor .


a single-sided parallel plate denuder with the impaction liquid acting as the sink, the predicted absorption becomes 53%. This clearly demonstrates that the mass transport to the impaction surface is primarily controlled by diffusion. These results show that gases that dissolve in the impaction liquid and form ions that may be found in the particle fraction must be removed prior to wet impaction. This is a minor obstacle, since both dry (Ferm, 1986) and wet (Simon and Dasgupta, 1993) denuders can be used to selectively remove gaseous components in air while leaving particulate material virtually unaffected.

Sample handling and analysis

Another factor that has to be considered to ascertain an accurate determination of the sampled species is evaporation of the impaction liquid. Depending on the air humidity, a varying amount of impaction liquid will be evaporated, leading to irreproducibility in the determinations. An enrichment step for capturing the total amount of analyte in the impaction liquid is therefore convenient, whereby the determination is based on the amount of substance, rather than the concentration in the impaction liquid.

In the first set-up of the sample handling system, the impaction liquid was conveyed directly from the WI to the enrichment column. However, this design demanded a high liquid flow, which caused back pulsation due to the back pressure of the enrichment column, disturbing the continuous flow of the liquid film on the impaction liquid support plate. The final design was to fill a separate vessel with the sample liquid, for subsequent emptying and enrichment. A relatively high flow was still needed to empty the container to achieve reasonable time resolution, and to reduce the back pressure 60 #m beads were chosen in the enrichment column.

This arrangement gave the sampling system a detection limit of 13pmolm -3 (1.24 ng m- 3) and 84 pmol m- 3 (5.21 ng m - 3) for particle-bound sulphate and nitrate in air, respectively, based on three times the standard deviation for five injections of the lowest standard. It should be pointed out that the chromatographic parameters were not opti- mised for high sensitivity, since the purpose of this experiment was to demonstrate the feasibility of coupling the WI on-line to an analytical system, not necessarily reaching the maximal sensitivity and lowest limit of detection.

Sampling of the atmospheric particle phase

This design has the limitation of only sampling a fraction of the atmospheric aerosol, and information of the fraction below 0.4 pm is missed. This is illustrated in Fig. 8a and b, showing typical distributions of particle-bound sulphate and ammonium in coarse mode and accumulation mode, measured by Hillamo et al. (1993) and Hillamo (1994), respectively, alongside the calculated sampled fraction of each distribution achieved by the WI. Variations in the chemical distributions occur of course between different locations and meteorological situations and these comparisons cannot be seen as general for all types of natural aerosols. For the sulphate distribution with a mass median aerodynamic diameter (MMAD) in accumulation mode at 0.5 ktm only 67% is captured, while the corresponding efficiency for the ammonium distribution is 83% with MMAD at 0.7 #m.

Reduction of the cut-off size

For collection of a total fraction of the accumulation mode the cut point of the impactor must be lowered, and at the conclusion of this work some experiments were carried out to investigate the possibility of reducing the cut-off size. In these experiments no liquid drain spout was used and only the impaction function was tested. The results showed that the impaction liquid was able to resist the elevated force influenced by the increased jet velocity corresponding to a cut-off of 0.2 #m. In spite of the small distance between the impaction liquid surface and the nozzle plate the impaction liquid film was sufficiently stable to allow continuous operation. The cavities formed in the impaction liquid surface became


0.45

0.4

o35 "~ 0.3

0.25 / 4 ~ 0.2

~ 0 . 1 5

O.ll /11 0.051 / / /

0 ~ ~ , . . . . . .

0.1 a) log D./~ln

0.35

0.3

"~ 0.25

0.2

~ 0 . 1 5

0.1

"= 0.05

0 • • • *

0. b) log D J ~ m

I Distribution of particle bound sulphur determined by Hillamo et al.

- o - - C u t offat 0.41

• ,,<>--Corresponding to a cut-off at 0.1p.m

10

Distributi ~,~ bound a ~ determim I by Hillamo

- . l ~ C u t offat ).41 gm

10

! ~ I I V B W

Fig. 8. Differential distribution curves for particle mass vs. particle size for the wet impactor compared with distributions of particle-bound sulphur and ammonium determined by Hillamo et al. (1993) and Hillamo (1994), respectively. The line describing the cut-offof at 0.1 #m in (a) is calculated

by transposing the 0.41 #m curve to this lower cut-off.

4 3:t ] - - 0 - - Sulfate ]

'it 0 . 5 ~

0 . . . . . I . . . . I . . . . . . . . . . . . . . I . . . . I . . . . I . . . . I . . . . I . . . . I . . . .

g~

348.8 348.9 349.0 349.1 349.2 349.3 349.4 349.5 349.6 349.7 349.8 349.9

Sample t ime/Jul ian date

Fig. 9. Ambient air concentration of particle bound sulfate and nitrate on particles between 0.41 and 3.0 #m between 14 and 15 December 1994 outside our laboratories in Stockholm.


no t i ceab ly deeper due to the increased pressure which increased the S / W ra t io and, p robab ly , would impa i r the size resolut ion. W h e n the air flow was fur ther increased co r r e spond ing to a cut-off of 0.1/~m the impac t i on l iquid film col lapsed. I t will hence become difficult to reach > 99% efficiency by le t t ing the par t ic les impac t in water , since the impac t i on film canno t wi ths tand the increased force induced from the jets. The c o m p a r i s o n of the sampl ing efficiency in the cut-off in terval and the par t ic le d i s t r ibu t ion d a t a f rom the l i t e ra ture presented in Fig. 8a, also shows tha t a cu t -o f fbe low 0.1/ t in would be necessary for > 99% col lect ion efficiency. Wi th these results we conc lude tha t it wou ld be imposs ib le to

sample the whole accumula t i on m o d e with this conf igura t ion and believe that a rad ica l ly new design would be requi red in o rde r for this to work.

Demons t ra t ion o f the W I / I C sys tem in ambient air measurement

The sampl ing system was tested in an ambien t air measu remen t s tudy dur ing a pe r iod of 24 h between the 14 and 15 of D e c e m b e r 1994. The par t ic le b o u n d su lpha te and n i t ra te were de t e rmined in the air outs ide the l a b o r a t o r y 2 k m nor th of S tockho lm City. The results can be seen in Fig. 9.

C O N C L U S I O N S

The W I presented makes it poss ib le to pe r fo rm a s ize-segregated par t ic le sampl ing and on-l ine de te rmina t ion . The ionic f ract ion of the par t ic le phase being sampled can be de t e rmined with high t ime resolut ion, which enables recogni t ion of var ia t ions in the par t ic le const i tuents . W i t h the se t -up demons t r a t ed , a sample frequency of 3.75 de t e rmina t ions per hou r of the i m p a c t i o n l iquid were used, y ie lding a t ime reso lu t ion and sensi t ivi ty that exceeds the previous s ize-segregated methods . A l though des igned to sample one f ract ion of part ic les , a modi f i ca t ion with a series of steps can be real ised for an increased size segregat ion. Such a design would fur ther increase the in fo rmat ion of the d i s t r ibu ted cons t i tuents in the par t ic le phase.

Acknowledgements--This work was supported by The Swedish Natural Science Research Council, through grant K-KU 8724-312, and by The Swedish Environmental Protection Agency. We are also indebted to Svante Jonsson for the skilful mechanical work in manufacturing the impactor unit, to G6ran Frank for help with calibrating the impactor, and to Kai Rosman and Tadeusz Ganko for assisting us with the WI measurements.

R E F E R E N C E S

Appel, B. R. (1993) Aerosol measurement--principles techniques and applications (Edited by Willeke, K. and Baron, P. A.). Van Norstrand Reinhold, New York.

Buhr, S. M., Buhr, M. P., Fehsenfeld, F. C., Holloway, J. S., Karst, U., Norton, R. B., Parrish, D. D. and Sievers, R. B. (1995) Atmos. Envir. 29, 2609 2624.

Cushing, K. M., McCain, J. D. and Smith, W. B. (1979) Environ. Sci. Technol. 13, 726-731. Ferm, M. (1986) Concentration measurements and equilibrium studies of ammonium, nitrate and sulphur species

in air and precipitation. Ph.D. thesis, Department of Inorganic Chemistry, Gothenburg University. Ganko, T. (1994) Institute of Applied Environmental Research, Air Pollution Laboratory, Stockholm University,

personal communication. Gudmundsson, A., Bohgard, M. and Hansson, H.-C. (1995) J. Aerosol Sci. 26, 915-931. Hansson, H-.C. and Nyman, S. (1985) Envir. Sei. Technol. 19, 1110-1115. Hillamo, R. E. (1994) Development of inertial impactor size spectroscopy for atmospheric aerosols. Ph.D. thesis,

Finnish Meteorological Institute, Helsinki. Hillamo, R. E., Kerminen, V.-M., Maenhaut, W., Jaffrezo, J.-L., Balachandran, S. and Davidson, C. I. (1993) Atmos.

Envir. 17/18, 2803-2814. Keskinen, J., Pietarinen, K. and Lehtimfiki, M. (1992) J. Aerosol Sci. 23, 353-360. Marple, V. A. (1970) Fundamental study of inertial impactors. Ph.D. thesis, University of Minnesota. Mercer, T. T. (1976) U.S. Atomic Energy Commission Report No. UR-3490-634. Nevalainen, A., Pastszka, J., Liebhaber, F. and Willeke, K. (1992) Atoms. Envir. 26, 531-540. Rader, D. J. and Marple, V. A. (1985) Aerosol Sci. Technol. 4, 141-156. Rubow, K. L. and Hillamo, R. E. (1994) Aerosol Sci. Technol. (accepted). Simon, P. K. and Dasgupta, P. K. (1993) Anal. Chem. 65, 1134-1139. Simon, P. K. and Dasgupta, P. K. (1995) Anal. Chem. 67, 71-78. Sneddon, J. (1984) Anal. Chem. 56, 1982-1986. Tropp, R. J., Kuhn, P. J. and Brock, J. R. (1980) Rev. Sci. Instrum. 51, 516-520.

Single-stage flowing liquid film impactor for continuous on-line particle analysis

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